JPS6358669B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for directional solidification of molten metal, and more particularly to an apparatus for producing single crystal articles having controlled crystal orientation.

It is well known that the performance of metal structures can be greatly improved by unidirectional casting methods that produce articles with columnar or single crystals. For example, such techniques are disclosed in US Pat. No. 3,260,505 and US Pat. No. 3,494,709. The primary purpose of prior art devices, methods, and articles is to improve properties along the major axis of the article, i.e., such that the solidification growth axis or axis of movement of the solidification interface is the same as the major axis of the article. The purpose was to provide a structure.

When a metal is directionally solidified, due to its nature, the metal often solidifies or grows faster in one crystal direction than in another crystal direction. For example, in nickel-based superalloys, the <001> direction has been found to be the preferred direction. Thus, single crystal cast articles formed by the means disclosed in the aforementioned U.S. Pat.
It has a direction. Special methods must therefore be used to obtain other crystallographic directions along the main solidification axis. Also, the orientation of the crystals with respect to the plane perpendicular to the solidification axis is almost always irregular, even in directionally solidified articles, unless controlled.
The crystallographic direction measured along the major axis of the cast article is referred to as the primary direction, and one polar axis direction in a plane perpendicular to the major axis is referred to as the secondary direction.

The properties of materials such as columnar or single crystal materials are influenced by their crystal orientation. For example, the modulus of elasticity of many alloys varies significantly, thus affecting the performance of the component under conditions of stress and strain. Control of the primary and secondary directions therefore becomes increasingly important when applying advanced materials to more advanced applications.
The crystal orientation of the material is determined by conventional non-destructive testing methods. For example, radiation diffraction using the Laue method is most useful. Furthermore, changes in crystal structure can be easily confirmed by conventional grain corrosion. If you know the direction of a certain member at a certain position,
If there are no intervening grain boundaries, the crystal orientation will be the same in other regions, and there will be no minute crystal changes beyond the range under consideration.

A useful method of controlling the crystal structure of a cast article is to
The first step is to use preformed metal species with the required texture. If an article can be cast by epitaxial growth from a seed, the crystal structure of the seed will be reproduced within the article.

Of course, it is well known to grow crystals from seeds to form articles. For example, U.S. Patent No.
The Bridgman process of producing single crystal articles by epitaxial growth, disclosed in No. 1,793,672 and other publications, was developed in the 1920's. US Pat. No. 2,791,813 discloses a structure with controlled crystal orientation, in which a seed crystal is used to obtain the required crystal orientation. U.S. Pat. No. 3,759,310 also discloses an apparatus and electric arc method for producing single crystal articles with consumable electrodes, in which a seed crystal placed at the bottom of the mold is used. . More recently, an improved method and apparatus for making single crystal articles is disclosed in US Pat. No. 3,857,436. This patent discloses a means and method for initiating crystallization in a conical lower chamber in which steep supercooling conditions are created. Further details are provided in US Pat. No. 3,857,436. U.S. Pat. No. 3,598,169 discloses casting a relatively flat object by using a wedge-shaped seed and solidifying in a diverging manner.

With the exception of the aforementioned US Pat. No. 3,759,310, the methods of all of the above-mentioned patents involve preheating the mold before introducing the molten metal. Also, in prior art methods, the seeds remain disposed within the mold during heating. Therefore, depending on the situation, the seeds may be heated to a relatively high temperature along with the mold, although the degree of heating may be smaller depending on the placement position of the seeds. Heated molten metal is introduced into and stored within the mold, and the molten metal contacts and partially melts the heated seeds. Of course, it is necessary to at least partially, but only partially, melt the seed, and thus there is a need to control the seed, mold, molten metal, and other factors during initial and transient conditions.

Many of the prior art techniques reflect experimental methods and are not suitable for mass production. There is now an increasing trend toward the commercial use of articles with controlled crystal structures, such as columnar and single crystal gas turbine blades. This has facilitated the development of automated casting methods that economically produce large quantities of articles. According to one method disclosed in US Pat. No. 3,895,672, a heated mold is clamped onto a cold chill plate immediately before introducing molten metal into the mold. If a seed crystal is used, the seed crystal is attached to a chill plate so that the seed crystal is cold. The short time it takes to join a hot mold and a cold chill plate provides little time for such a temperature increase. Similar difficulties exist with some prior art devices and methods. If the seeds are too cold, sufficient melting will not occur and no epitaxial growth will occur. One way to overcome this is to significantly overheat the molten metal. However, such methods are undesirable because overheating increases time and cost, causes undesirable evaporation of the elements, and often causes increased deterioration of the mold. Also, heating the seeds separately or placing them in the mold while the mold is being heated, as per prior art methods, is mechanically and manufacturingly complex, and may result in excessive seeding. It is not preferred because it may be oxidized or contaminated.

Another point that must be taken into account when manufacturing articles with controlled primary and secondary crystal orientation is that after manufacturing the seed orientation is first accurately determined by suitable inspection methods and then cast. This means that the axis of the article must be precisely controlled. Therefore, preparing seeds for casting can be quite expensive. Therefore, after the article is formed, it is desirable that the seed be recovered from the casting process and, if possible, reused.
However, if the seed is significantly melted during the casting process or is surrounded by a large amount of solidified metal with disparate orientations, it is difficult to recover it for reuse.

It is an object of the present invention to provide improved methods, apparatus, and molds for producing cast articles with controlled crystallographic orientation by epitaxial growth from seeds with known crystallographic orientation. Another object of the invention is to preserve, recover and reuse seeds.

According to the invention, the molten metal is oriented in a manner such that a portion of it flows over the seeds, heating and melting the portion and also removing any undesirable contaminating film that may be present. Introduced into a solidification mold. When used in conjunction with a chill plate, the mold defines a starter cavity having a volume sufficient to accommodate the seeds and to receive molten metal flowing over the seeds. It is formed into a shape. The seed may protrude into the starter cavity so that molten metal can flow around and heat the seed. A barrier layer, such as a ceramic coating, may be provided on selected portions of the seed to facilitate removal of the seed from the solidified dissimilar metal casting for reuse. In one embodiment, thermal treatment is applied to retard uncontrolled directional solidification of molten metal within the starter cavity and to ensure that epitaxially solidified metal from the seed is present in the article. An insulating layer is disposed on the chill plate in proximity to the seeds.

In one embodiment of the invention, the mold has an article section connected to a starter section by a selector section. The starter section is configured to provide a volume capable of containing the seed and receiving a portion of the molten metal flowing around the seed to heat and melt it. The selector section is located in close proximity to the area of the starter section that can receive the seeds, and prevents only the solidification interface of the metal that is being epitaxially solidified from the seeds to pass into the article section. It does the work of forgiving. In one preferred embodiment, the mold is configured to receive molten metal through the article section, and the transfer of the molten metal from the selector section to the starter section causes the molten metal to impinge on the seed surface. This effectively heats and controls the melting of the seeds.

The present invention is suitable for making cast articles from any alloy with any desired controlled structure that can be made from seed. Among other things, an important application is the production of columnar or single crystal components of nickel superalloys.

The present invention properly melts the seed to ensure that the desired epitaxial growth occurs from the seed;
This avoids the production of defective cast articles that can be produced if the seed is not sufficiently fused or the contaminant layer is not sufficiently removed. Further, the present invention allows the use of seed crystals that are not substantially heated before the molten metal is introduced into the mold. In one preferred embodiment, seed costs are further reduced by making the solidified cast article more easily recoverable and subsequently reusable. The use of seeds has become more economical and therefore more convenient than growing without seeds, and benefits are gained by controlling the primary and secondary directions. The design of the single-crystal mold is simplified and the initial solidification rate is increased, which allows for increased production.

The invention will now be described in detail with reference to preferred embodiments thereof, with reference to the accompanying drawings.

A preferred embodiment of the present invention is described for a mold for producing monolithic single crystal nickel alloy castings constructed for use within the disclosure of the aforementioned U.S. Pat. No. 3,895,672;
This embodiment is not limited to such applications.

A mold 20 made of a ceramic material suitable for forming single crystal articles is placed on a steel chill plate 22 as shown in FIG. The mold includes a section defining an article cavity 24, which, as shown in FIG. formed into a shape. Mold 20 also has a first end 26 for receiving and directing molten metal into the article cavity and a second end 33 configured to contact a chill plate.

A seed 28 having a predetermined crystal orientation is mounted within a recess 30 formed in the chill plate 22. This seed 28 is in intimate contact with the chill plate and is adapted to be cooled by the chill plate. A starter cavity 32 defined by the second end 33 of the mold or starter section and the chill plate 22 surrounds the seed 28. A selector section 34 connects starter cavity 32 and article cavity 24. The selector section 34 has a substantially smaller cross-sectional area than either the starter cavity 32 or the article cavity 24.

In the preferred embodiment shown, the seed, starter cavity, and selector section are circular in cross-section, but other cross-sectional shapes will work just as well.

The relative sizes of the respective elements are not constant, but may be in a relative relationship such that a general idea can be given by the following example. For example, the height is 10
When producing nickel superalloy articles such as ~25 cm gas turbine engine blades, the superalloy seeds are preferably 2 to 2.5 cm in diameter and about the same length as the height. . The diameter of the starter cavity is preferably about 5 cm, and the entrance to the selector section is preferably located about 0.5 to 1.0 cm from the upper surface of the seeds. The starter cavity thus has a volume that is more than five times the volume of the seeds contained therein. As discussed below, this volume can be used to receive molten metal to heat the seed and initiate epitaxial solidification from the seed.

The end 33 of the starter section is disposed in a fluid-tight manner on the surface 36 of the chill plate to prevent leakage of molten metal. Clamping means, illustrated as bolts in FIG. 3, are used to maintain the mold and chill plate in good contact. Other mechanical fastening or securing means may be used as well, as long as they are arranged outside the molten metal flow path and are configured to hold the mold at high temperatures. Of course, in mass production, the criterion in selecting such clamping means is the ease and speed with which it can be installed or removed.

Once the mold and chill plate are firmly clamped together, the resulting assembly can be placed in various prior art devices for directional solidification. Molten metal is introduced and the required temperature gradient is applied to the mold to directionally solidify the casting. The usage of the device is as follows. Molten metal is introduced into the mold 20 through a receiving end (first end) 26 from which it passes through the article section and the selector section to impinge on and cross the surface 38 of the seed 28. Cut and flow. The action of the molten metal on the seed surface 38 heats and melts the seed;
The purpose of stirring is to promote the removal of deposits and films adhering to such surfaces. Molten metal flowing across the surface of the seed collects in starter cavity 32 adjacent to the seed. The starter cavity thus serves as a receiving reservoir for the molten metal used to heat the seeds. Such a receiving reservoir may be located at a separate location from the cavity containing the seeds, if desired. Metals may also be introduced through isolation gates as disclosed in US Pat. No. 3,915,761. In such cases, the starter cavity must be shaped in such a way that molten metal can flow through it.
Each element is shaped as shown in the preferred embodiment of FIG. 1 so that after the molten metal has passed over the surface of the seed, it surrounds the sides of the seed and thereby It also gives heat to the seeds.

Once the mold is filled with molten metal, the molten metal is gradually allowed to solidify along the major axis of the mold, ie, vertically, by removing heat through the chill plate and the walls of the mold in a manner well known in the art. . The metal in the starter section solidifies first. Of course, the main part of a species always exists as a solid. Since the selector section 34 is arranged above the seed 28 with its center aligned, the solidification interface of the metal epitaxially solidifying on the surface of the seed first reaches the selector section 34 and displaces the section. pass. Since the metal that solidifies while passing through the selector section 34 is epitaxially solidified from the seed crystal, the solidified metal has the same crystal direction as the seed crystal. Similarly, the article formed in the article cavity 24 follows the crystal structure of the material already solidified in the selector section, and therefore has a similar crystal orientation.

FIG. 3 is a schematic cross-sectional view showing in more detail the arrangement of important elements of the invention in the starter section. In order to obtain the required secondary orientation, it is necessary that the seed crystal be oriented in some predetermined manner with respect to the article cavity 24. This is accomplished by orienting the seeds and mold in a fixed relationship to the chill plate 22. As shown in FIG. 3, the mold is fixedly oriented to the chill plate 22 by bolts 37, which also serve to clamp the mold to the chill plate to prevent leakage of molten metal. . Particularly in mass production, orientation can be determined by forming chill plates and molds into specific shapes at their contact points, or by using electro-optic sensors with suitable pointers, etc. Other orientation means can of course also be used. The means for accurately orienting the seeds relative to the chill plate is illustrated in more detail in FIG. Orientation along the vertical or primary axis is achieved by secure means of placing the seeds on the surface of the chill plate. Secondary directions around the main axis or directions perpendicular to the main axis are controlled by mutually engaging grooves and keys. As shown, the seed crystal has a simple groove 41 extending across its diameter, and the chill plate has an integral key 48. Other mechanical restraint means, positioning means and other orientation methods may also be employed.

Ceramic shield 4 surrounding the periphery of seed 28
0 is illustrated in FIGS. 3 and 4. This shield 40 is a barrier layer that prevents molten metal flowing over the seed surface 38 and introduced into the starter cavity 32 from adhering to the seed periphery 42. This shield 40 prevents the seed periphery 42 from melting and also prevents molten metal in the starter cavity from adhering to the seed periphery 42. Thus, after the metal in the starter cavity 32 has solidified and the entire casting has been removed from the mold, the casting can be cut transversely to the plane of the surface 38, thereby removing the starter for the casting. Seeds can be removed more easily than sections, and the seeds can be reused with little modification.

FIG. 5 shows another embodiment of a ceramic shield 40 fitted into a chill plate with seeds. This shield may be constructed of a ceramic material or any other material capable of resisting the action of the molten metal for a short period of time during which it is exposed to the metal before solidifying. Therefore, it is sufficient that such a shield is made of a material that has the requisite heat resistance and corrosion resistance as described above, and also has sufficient mechanical strength that it will not become warm even under the action of molten metal. be. Of course, to achieve the objectives of the present invention, the barrier layer around the periphery of the seeds need not be a separate physical element, but may be a coating applied over the seeds. FIG. 6 shows yet another embodiment of the invention in which the seeds are mounted with shield 40 at the same height as the recessed area on the surface of the chill plate. This embodiment includes an annular ceramic disk 44 mounted on the surface 36 of the chill plate proximate the seeds. This disk 4
4 has the function of reducing the degree of cooling by the chill plate and thus the rate of solidification of the molten metal in the vicinity of the seeds to a value smaller than that which would occur if such a disk were not provided. The metal that originally solidifies from the surface 36 of the chill plate does not have the required crystal orientation of the seed. If the starter cavity is formed in a particular shape, the presence of the disk 44 may prevent the metal epitaxially solidifying from the seed surface 38 from reaching the selector section 34 in an undesired crystal orientation. is prevented from reaching the selector section 34. In FIG. 6, disk 44 is shown as a separate element covering the exposed chill plate within starter cavity 32. However, the diametrical extent of the shroud may be varied, such as by reducing the diameter of the disc 44, so that a portion of the surface of the chill plate is exposed at the periphery of the starter cavity. Varying the coverage of the chill plate controllably varies the degree to which heat is removed from the metal within the starter cavity 32 to achieve the desired solidification of the article. Additionally, disk 44 may be integrally formed with shield 40 as shown in FIG. Furthermore, the disk 44 may be integrally formed with the mold 20, in which case the inner diameter of the disk portion may be varied to control heat extraction. The disk 44 may be formed as a coating on the chill plate;
In this case, the function of the disk is varied by the thickness and thermal properties of its material.

The apparatus and methods disclosed herein can be used to manufacture a single part or several parts. Of course, as is commonly done in the mass production of directionally solidified precision castings, a plurality of cast articles may be produced by arranging a plurality of molds of the type shown in FIG. can do. Alternatively, more than one member may be formed from a single seed crystal by branching the mold directly above the selector section as disclosed in US Pat. No. 3,857,436.

Although the invention has been described in terms of preferred embodiments for producing single crystal cast articles, it is within the scope of the invention to produce columnar crystal castings and other epitaxially solidified cast structures. It is. The invention is also operable with any casting alloy for which suitable molds can be made. Furthermore, although the present invention has been described in detail with reference to several embodiments thereof, the present invention is not limited to these embodiments, and various modifications and omissions may be made within the scope of the present invention. It will be obvious to those skilled in the art that this is possible.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing a mold containing seeds placed on a chill plate. FIG. 2 is a schematic cross-sectional view showing a cross section of the article cavity of the mold shown in FIG. FIG. 3 is a partial sectional view showing the starter cavity of the mold on the chill plate. FIG. 4 is an illustrative enlarged partial sectional view showing the arrangement of seeds.
FIG. 5 is a schematic partial cross-sectional view showing a species having a barrier layer around its periphery. FIG. 6 is an illustrative partial cross-sectional view showing another embodiment of the barrier layer of the seed and chill plate. 20...mold, 22...chill plate, 2
4...Article cavity, 26...First end, 2
8... seed, 30... recess, 32... starter cavity, 33... second end, 34... selector section, 36... surface, 37... bolt,
38...Surface, 40...Shield, 41...Groove,
42...periphery, 44...disc, 46...groove,
48...Key.

Claims (1)

[Scope of Claims] 1. An apparatus for solidifying molten metal into an article having a controlled crystal orientation, comprising: a mold; a chill plate for cooling the molten metal while the molten metal is directionally solidifying; and an article. a seed having a known crystallographic orientation to initiate the desired epitaxial solidification; means for retaining said seeds in a predetermined orientation in proximity to said chill plate in a mold; and means for holding said mold relative to said chill plate such that said mold is in a predetermined orientation relative to said seeds. the mold has an article section, a starter section, and a selector section located therebetween, the starter section contacting the chill plate and holding the chill plate of the species together with the chill plate; Apparatus characterized in that a volume is defined around the more protruding part and the seed, which is capable of receiving a certain amount of molten metal.